1. Field of the Invention
The present invention relates to a method for forming an isolation film in a semiconductor device, and more specifically, to a method for forming a self-aligned shallow trench pattern in a NAND flash device.
2. Discussion of Related Art
Generally, in order to form a transistor on a semiconductor substrate, an active region that is electrically electrified and an isolation region for preventing electrical electrification and isolating elements are formed in the semiconductor substrate. A portion of the semiconductor substrate in the isolation region is etched and is then filled to form an isolation film.
Conventionally, after the semiconductor substrate in the isolation region is etched, a post etch treatment (PET) process for a thin etch of a trench sidewall is implemented. At this time, an abnormal layer is formed due to polymer of a carbon series generated from the PET process, thus resulting in uneven oxidization in a subsequent oxidization process.
FIG. 1A and FIG. 1B are TEM photographs for explaining sidewall damage by a conventional process.
Referring to FIG. 1A and FIG. 1B, after a trench for isolation is formed, if a post cleaning process is performed, the semiconductor substrate exposed by the etch process is damaged. Damage of the semiconductor substrate within the trench adversely affects the properties of the isolation film.
FIG. 2 is a TEM photograph showing that the abnormal layer of the carbon series is formed after the PET process in the related art.
FIG. 3 is a TEM photograph for explaining the disparity of a sidewall oxide film in the related art.
FIG. 4 shows a TEM EDS analysis table for the sidewall oxide film in the related art.
Referring to FIG. 2 to FIG. 4, if a trench is formed through an isolation etch according to a conventional isolation process and a PET process is performed, an abnormal layer of a carbon series is formed within the trench.
If the inside of the trench wherein the abnormal layer is formed is experienced by a sidewall oxidization process, a very uneven sidewall oxide film is formed. Accordingly, there occurs a problem that a target trench shape is difficult to form. It is also impossible to reduce damage due to the etch.